The present invention relates to a printer used in point-of-sale (POS) terminals, cash registers, photocopiers, and facsimiles, and to a paper carrying mechanism therefor.
For example, Japanese Unexamined Patent Application Publication No. 9-240083 discloses a printer having a structure such that an upper frame is rotatably attached to a base frame and the upper frame has a platen and so on provided therein. This type of printer generally employs one of the following first and second drive systems.
First, a first drive system will be described with reference to FIG. 1.
An upper frame 32 is attached to a base frame 31 with a fulcrum shaft 33 rotatably rightward by a slight angle from the shown position. The base frame 31 includes a drive motor 34, a drive gear 35 fixed to a rotating shaft (not shown) of the drive motor 34, a first reduction gear 36 engaged with the drive gear 35, and a second reduction gear 37 coaxial with the first reduction gear 36.
The upper frame 32 includes a platen gear 38 and a platen 39 coaxial with the platen gear 38.
When the drive motor 34 rotates, the torque is transmitted to the platen 39 via the drive gear 35, the first reduction gear 36, the second reduction gear 37, and the platen gear 38.
In the first drive system, the distances between the shaft of the drive gear 35, the shaft of the first reduction gear 36 and the second reduction gear 37, and the shaft of the platen gear 38 are not maintained constant. Therefore, the amount of backlash between the drive gear 35 and the first reduction gear 36, and the amount of backlash between the second reduction gear 37 and the platen gear 38 are not stable. Therefore, pitch fluctuations tend to occur in the printing.
Next, a second drive system will be described with reference to FIG. 2.
An upper frame 42 is attached to a base frame 41 with a fulcrum shaft 43 rotatably rightward by a slight angle from the shown position. The base frame 41 includes a drive motor 44, a drive gear 45 fixed to a rotating shaft (not shown) of the drive motor 44, and a first reduction gear 46 engaged with the drive gear 45.
The upper frame 42 includes a second reduction gear 47, a reduction pulley 48 coaxial with the second reduction gear 47, a platen pulley 50, a belt 49 stretched around the reduction pulley 48 and the platen pulley 50, a platen 51 coaxial with the platen pulley 50, and a tension roller 52 applying tension to the belt 49.
When the drive motor 44 rotates, the torque is transmitted to the platen 51 via the drive gear 45, the first reduction gear 46, the second reduction gear 47, the reduction pulley 48, the belt 49, and the platen pulley 50.
In the second drive system, in order to deal with a variation in the length of the belt 49 and to maintain the tension of the belt 49 constant, a tension roller 52 is adopted. However, this tends to complicate the structure. In addition, this does not solve the above-described problem concerning the amount of backlash.